Cavity enhanced optical spectroscopy (CEOS) entails the use of a passive optical resonator, also referred to as an optical cavity, to improve the performance of an optical spectroscopy instrument. Cavity enhanced absorption spectroscopy (CEAS), and cavity ring down spectroscopy (CRDS) are two of the most widely used CEOS techniques. The intensity of single-mode radiation trapped within a passive optical resonator decays exponentially over time, with a time constant τ which is often referred to as the ring-down time. In practice, it is preferable to ensure that only a single resonator mode has an appreciable amplitude, since excitation of multiple resonator modes leads to multi-exponential radiation intensity decay (i.e., multiple time constants), which significantly complicates the interpretation of measurement results. The ring-down time τ depends on the cavity round trip length and on the total round-trip loss within the cavity, including loss due to absorption and/or scattering by a sample present in the cavity. Thus, measurement of the ring-down time of an optical resonator provides spectroscopic information on a sample within the resonator, and both CRDS and CEAS are based on such a measurement of τ.
In CRDS, an optical source is coupled to the resonator in a mode-matched manner, so that the radiation trapped within the resonator is substantially in a single spatial mode. The coupling between the source and the resonator is then interrupted (e.g., by blocking the source radiation, or by altering the spectral overlap between the source radiation and the excited resonator mode). Typically, a detector is positioned to receive a portion of the radiation leaking from the resonator, which decays in time exponentially with time constant τ. The time-dependent signal from this detector is then processed to determine τ (e.g., by sampling the detector signal and applying a suitable curve-fitting method to a decaying portion of the sampled signal). Note that CRDS entails an absolute measurement of τ. The articles in the book “Cavity-Ringdown Spectroscopy” by K. W. Busch and M. A. Busch, ACS Symposium Series No. 720, 1999 ISBN 0-8412-3600-3, including their cited references, cover most currently reported aspects of CRDS technology.
Single spatial mode excitation of the resonator is also usually employed in CEAS, (sometimes called integrated cavity output spectroscopy (ICOS)), but CEAS differs from CRDS in that the wavelength of the source is swept (i.e., varied over time), so that the source wavelength coincides briefly with the resonant wavelengths of a succession of resonator modes. A detector is positioned to receive radiation leaking from the resonator, and the signal from the detector is preferably integrated for a time comparable to the time it takes the source wavelength to scan across a sample spectral line of interest. The resulting detector signal is proportional to τ, so the variation of this signal with source wavelength provides spectral information on the sample. Note that CEAS entails a relative measurement of τ. The Ph.D. dissertation “Cavity Enhanced Absorption Spectroscopy”, R. Peeters, Katholieke Universiteit Nijmegen, The Netherlands, 2001, ISBN 90-9014628-8, provides further information on CEAS and CRDS technology and applications. The Peeters dissertation focuses primarily on the use of lasers (either pulsed or CW) as the light source for CEOS instruments. However, an article by Fiedler et. al., Chemical Physics Letters 371 (2003) 284–294 teaches that an incoherent light source e.g., a xenon arc lamp is also suitable for cavity-enhanced absorption spectroscopy. Other incoherent sources are also suitable e.g., LEDs.
Independent of the light source selected, achieving maximum performance from CEOS instruments requires that the optical cavity (and preferably the entire optical train) be maintained at a uniform and stable temperature with minimal environmental perturbation (e.g., vibration) during the course of an analysis. Temperature sensitivity of the operating frequency is characteristic of all electromagnetic and acoustic resonators, including lasers, due to thermally induced variations in the size, dielectric constants, speed of sound, etc., for solid-state materials. Fractional variations of these parameters is typically 10−4 to 10−5 parts per degree Kelvin. If the sample being analyzed is a liquid, its index of refraction will change with temperature which change can produce drift and/or inaccuracy in measurement. When CRDS or CEAS is being used to determine gas isotope ratios e.g., C14O2 vs. C12O2, a change in temperature can affect the Boltzman distribution for the different isotopes and hence change the measured isotope ratio for a fixed composition.
There is currently a need for a highly sensitive and accurate CEOS instrument that is also sturdy, portable and can be moved relatively easily so as to permit, for example, multi-location pollution monitoring or explosive detection. Such an instrument may be exposed to a rapidly varying ambient temperature, as well as other environmental perturbations such as vibrations, so isolating a CEOS instrument from the ambient environment, both thermal and vibrational, is highly desirable. Although current CEOS instruments frequently place the optical train, or at least the optical cavity, in some type of enclosure to reduce the influence of the ambient environment, current designs are inadequate in these respects.
U.S. Pat. No. 5,692,556 describes a temperature controlled test chamber for electronic components where the air temperature variation in the chamber can reportedly be maintained in the milli-Kelvin range. A critical aspect of the design of this chamber is ensuring a turbulent air flow pattern throughout the chamber. The basic premise is that by moving the air as rapidly as possible throughout all parts of the enclosure in a turbulent flow, a uniform temperature will be achieved throughout the chamber. While it is true that turbulent air flow within a chamber generally affords low internal temperature gradients, such turbulent flow also tends to rapidly transfer heat from (or to) the ambient air outside the enclosure to the internal components. As indicated, a critical aspect of CEOS is that the temperature of the instrument remain constant during the course of an analysis. It is less important that it remain at a particular temperature. In addition to the inherent problem of the impact of rapidly moving air on delicate optical components within the enclosure, (note that, in general, the greater the flow rate, the greater the level of impact) we have found that a turbulent flow approach is incapable of maintaining the temperature within the enclosure (and hence the temperature of the optical train) within the very narrow range necessary to obtain the maximum sensitivity and selectivity of which CRDS or CEAS is capable. For utmost accuracy in spectroscopic measurement, we have found it desirable to maintain the temperature of the optical train within the range of ±0.01 K, more preferably ±0.001K. Existing enclosure designs have not demonstrated this level of performance.